The present invention pertains generally to tester fixture design of printed circuit board tester fixtures, and more particularly to techniques for determining locations of maximum deflection of a printed circuit board (PCB) under test to be mounted in a tester fixture of a printed circuit board tester.
Printed circuit assemblies (PCA's) must be tested after manufacture to verify the continuity of the traces between pads and/or vias on the board and/or to verify that any components loaded on the PCA perform within specification.
Printed circuit assemblies testing requires complex tester resources. The tester hardware must be capable of probing conductive pads, vias and traces on the board under test.
Prior art test fixtures typically employed a bed of nails fixture comprising a large number of nail-like test probes having tips that make electrical contact with the nodal points of the circuit to be tested. The test probes are typically spring loaded pins inserted in receptacles that pass through and are secured relative to a supporting plate, hereinafter called the probe plate. The printed circuit is placed on top of the test probes and sealed with a gasket. A vacuum is applied through the test fixture to draw the printed circuit board down onto the spring loaded test probes to ensure good electrical contact. The vacuum is maintained until the testing is complete after which another printed circuit board is placed onto the test fixture for testing.
The test probes are inserted into the receptacles which extend below the lower side of the probe plate. The lower end of the receptacle typically has a wire wrap post. A wire is wrapped about the receptacle post and extends in a point-to-point wiring connection to an interface connector pin inserted into a fixture interface panel. The fixture interface panel is adapted to be connected to an interface receiver of the test electronics analyzer. The point-to-point wiring of each receptacle post to a corresponding interface connector pin involves manually wire wrapping wires between each of the receptacle post and interface connector pin.
With the miniaturization of electrical components, the number of test points in a circuit has risen significantly, making point-to-point wiring for each fixture a labor intensive operation. Automation of this process is not economical because each printed circuit board requires a unique design configuration. Furthermore, as the number of test points increases with shrinking technology, the wiring of the closely adjacent test pins becomes more tedious.
Recent advances in fixture technology has led to the use of wireless fixtures. In a wireless fixture, a fixture printed circuit board (PCB) replaces the wires connecting the tester interface pins to the fixture probes with traces on a printed circuit board. In particular, the tester interface pins (either directly or indirectly through tester adapter probes interfacing to the tester pins) make electrical contact with conductive pads on the bottom side of the fixture PCB. The conductive pads on the bottom side of the fixture PCB electrically connect to conductive pads on the top side of the fixture PCB through traces and vias. One end of the probes of the fixture probe plate makes electrical contact with the conductive pads on the top side of the PCB, while the other end of the probes of the fixture probe plate makes electrical contact with various conductive pads/nodes of the printed circuit board under test. Accordingly, wireless test fixture allows tester pins to make electrical contact with appropriate nodes of the printed circuit board under test without the necessity and complexity of wire-wrap connections.
When a printed circuit board under test is to be tested using a wireless fixture, the tester interface pins (directly or indirectly through tester adapter probes) press on the fixture PCB upward at its bottom conductive pads. Simultaneously, the bottom tips of the fixture probes press against the fixture PCB downward against its top conductive pads. The top tips of the fixture probes press against the bottom conductive pads of the printed circuit board under test. Connectivity problems can occur with this configuration. To this end, if each force exerting on one side of the wireless PCB is balanced along the same line by an equal force exerting on the other side, then the wireless PCB will be subject to compression at the points of application of the forces with no deflection. However, in areas where the force on one side is not balanced by another force on the other side, the wireless PCB will tend to deflect or warp in one direction. Deflections and warps can render the wireless PCB mechanically or electrically defective.
Accordingly, it is advantageous to insert spacers and/or standoffs (also known as “retainer screws”) (collectively referred to herein as “supports”) in strategic positions on the wireless PCB to prevent the occurrence of PCB deflection and/or warping. In particular, spacers may be inserted to provide support on the top side of the wireless PCB to balance forces from the tester pins (directly or indirectly through the tester adapter probes) at the bottom side of the PCB when there are not enough fixture probes exerting force on the top side of the PCB to provide this balancing. Conversely, standoffs may be inserted to balance the force from fixture probes against the operating side of the PCB when there are not enough from the tester pins (directly or indirectly through tester adapter probes) at the bottom side of the PCB. As known in the art, a standoff is a two-part piece comprising a screw and a threaded socket or hole.
This problem occurs more often than not due to the fact that while the tester interface pins (and/or tester interface adapter probes) are generally arranged in a fixed predetermined pattern that does not change from one PCB design to another, the pattern and number of fixture probes does tend to change from PCB design to PCB design. Typically, there are fewer fixture probes exerting downward force on the top side of the wireless PCB than tester interface pins (and/or test interface adapter probes) exerting upward force on the wireless PCB. Accordingly, spacers and/or standoffs are needed in those areas of the wireless PCB where the downward and upward forces are out of balance.
The placement of spacers and/or standoffs can be problematic. Inserting a spacer and/or standoff at every possible location that the forces of the tester interface pins (and/or tester interface adapter probes) and fixture probes do not counterbalance each other is not a viable solution because this may require drilling several hundreds of holes in the fixture probe plate and inserting hundreds of spacers and/or standoffs when only a few are necessary. This unnecessarily increases the cost and complexity of the fixture.
Accordingly, a need exists for intelligently determining the placement of spacers and/or standoffs for a printed circuit board. More generally, a need exists for determining locations of maximum deflection of a PCB under test when mounted in a tester fixture of a printed circuit board tester and for allocating fixture components at those locations in order to offset and reduce the magnitude of deflection of the PCB.